Here's an example of a rain core with Mawile (inspired by Scar's team; in theory it looks good, but I don't know how good it actually would be in battle):
Tornadus is a great partner for Kyogre on a dedicated (or even non-dedicated) rain team, as its Role Play allows it to copy Kyogre's Primordial Sea and set up rain again even after Groudon or Mega Rayquaza are sent out. Tailwind also allows it to sweep more effectively when not under Trick Room.
Rain teams are definitely very viable this year and, honestly, I prefer them over sun teams.
As mentioned before, Pokemon like Scizor, Ferrothorn, and Mawile can work especially well under rain. Politoed and Rain Dance Thundurus, though, aren't the best choices. I recommend reading up on the Primals' (and Ray-M's) weather mechanics to see why.